Orthogonal frequency division multiplexing is an efficient technique for high speed data transmission such as that needed for wireless data systems. Orthogonal frequency division multiplexing is a form of multicarrier modulation.
Multicarrier modulation systems with orthogonal sub-carriers having overlapping frequencies have been known since the late 1960s. R. W. Chang first described such a system in an article in the Bell Systems Technical Journal entitled "Synthesis of Band-Limited Orthogonal Signals for Multicarrier Data Transmission," pp. 1775-1796, December 1966. A similar system was described by B. R. Saltzberg in "Performance of an Efficient Parallel Data Transmission System," IEEE Transactions on Communication Technology, Vol. COM-15, No. 6, pp. 805-811, December 1967. In these systems, each sub-carrier overlapped only its two nearest frequency neighbors. The orthogonality between the overlapping sub-carriers was maintained by staggering, or time offset, of the in-phase and quadrature-phase components (so-called staggered quadrature amplitude modulation).
In 1971, an orthogonal frequency division multiplexing system using fast Fourier transforms to generate orthogonal wave forms was described in an article by S. B. Weinstein and P. M. Ebert entitled "Data Transmission by Frequency Division Multiplexing Using the Discrete Fourier Transform," IEEE Transactions on Communication Technology, Vol. COM-19, No. 5, pp. 628-634, October 1971. In this type of orthogonal frequency division multiplexing system, the data symbols are processed in the transmitter by an inverse fast Fourier transform and in the receiver by a fast Fourier transform. The symbols are time-limited and all sub-carriers overlap each other in the frequency domain. In such systems wherein data is transmitted in parallel, longer symbol intervals are possible, reducing sensitivity to impulse noise and providing high spectral efficiency by frequency overlapping. Advantageously, long symbols reduce the problems of inter-symbol interference. Accordingly, orthogonal frequency division multiplexing is an extremely effective technique for combating multipath fading such as that encountered over mobile wireless channels.
Orthogonal frequency domain multiplexing systems are being proposed for or used in wireless data transmission in a wide variety of applications including high definition television, cellular mobile telephony, and personal communications systems (PCS). It is in the area of mobile communication systems (i.e., cellular phones) that the benefits of orthogonal frequency division multiplexing appear to make it an optimal solution to the problems of limited bandwidth.
Orthogonality is a property of a set of functions such that the integral of the product of any two members of the set taken over the appropriate interval is zero. For example, trigonometric functions appearing in Fourier expansions (e.g., sines and cosines) are orthogonal functions. Orthogonality is desirable in communication because orthogonal channels or frequencies are statistically independent and do not interfere with each other, allowing for greater bandwidth density.
Orthogonality ensures that a receiver demodulating a selected carrier demodulates only that carrier without simultaneously and unintentionally demodulating the other carriers that are providing parallel data transmission along the multiplexed communication channel. Accordingly, there is no cross talk between carriers even though the carrier spectra overlap and there is no requirement of explicit filtering.
Orthogonal frequency division multiplexing systems may be subject to co-channel interference in a cellular mobile communications environment in which frequency reuse considerations result in the same carrier frequency being used for different communications in adjacent or nearby cells. Co-channel interference occurs when the same frequency is allocated to different conversations or data communications in adjacent cells in a cellular system such as used for mobile telephones. For those systems that are subject to co-channel interference, adaptive antenna arrays are considered highly desirable to suppress the co-channel interference. For example, adaptive antenna arrays have proved effective at suppressing co-channel interference and mitigating rapid dispersive fading and accordingly increasing channel capacity in Time Division Multiple Access (TDMA) systems.
Adaptive antenna arrays are capable of providing optimal gain as well as minimizing interfering signals. An adaptive antenna array typically includes an array of two or more elements with outputs that can be adaptively combined to control signal reception. The antenna elements can be arranged in linear, circular or planar configurations and are normally installed at base stations, although they may also be used in mobile telephones or portable computers. Adaptive antenna arrays are effective in reducing co-channel interference.
However, utilization of an adaptive antenna array is best accomplished using a minimum mean-square-error (MMSE) diversity combiner (DC), in which case it is necessary to estimate certain parameters for the minimum mean-square-error diversity combiner as they will not be known and accordingly must be estimated. The signals received by the separate antennas are independently demodulated before being applied to a diversity combiner.
Each received symbol is then evaluated based on a minimum mean square error. The minimum mean square error is a measure of the error probability in a signal based upon the mean value of the square of the error, (y-y).sup.2, where y is any estimate of y. In general, if y is any random variable having a mean value of y, then choosing y=y minimizes the mean square error. Of course, without having the values of y it is impossible to precisely evaluate y. Rather, it is necessary to estimate the mean value of y, y, based on supplied parameters. The accuracy and usefulness of the estimated mean square error is related to the quality of the parameters used in the estimation thereof.
In an adaptive antenna array, selection of appropriate parameters that accurately describe channel characteristics will minimize the mean square error of the received signal and accordingly decrease the signal error rate, effectively suppressing interference and maintaining signal quality.
A diversity combiner is used for diversity reception in an adaptive antenna array. Diversity reception is essentially multiple reception of transmitted information (i.e., from two or more antennas, the signals from which are separately demodulated) in order to reduce transmission errors and accurately receive the transmitted information. Diversity reception typically reduces or eliminates drop outs caused by multi-path phase cancellations (i.e., multi-path fading). Diversity combining also produces gain (relative to diversity switching or non-diverse reception) by effectively combining the power of multiple received signals. The signal to noise ratio (and the signal to interference ratio) may also be maximized.
It is considered desirable to implement an orthogonal frequency division multiplexing communication system in a digital cellular telephone network in order to provide resistance to intersymbol interference and thus better enable interference rejection. However, co-channel interference undesirably limits the performance of such a system. Adaptive antenna arrays offer a solution to the problem of co-channel interference in an orthogonal frequency division multiplexing system, but an adaptive antenna array is only viable if the parameters needed to optimize performance to reliably extract the transmitted information can be readily obtained.